This invention relates generally to the field of electrical supplies; and, in particular, to a charge pump circuit for avoiding excessive discharge of the current available at the output.
Low voltage integrated circuitry has steadily improved over the years. Presently, low voltage integrated circuit devices commonly operate in the two to three volt range. Low voltage operation provides, among other benefits, low power consumption. Thus, in battery operated devices, such as portable telephones, pagers, lap-top computers, hot swap devices and the like, low voltage integrated circuitry allows the devices to operate proportionally longer than devices operating at higher voltages.
Low voltage operation, while providing many benefits, causes problems with respect to some of the circuitry contained in the integrated circuit. Field effect transistors, which are commonly used for switching, require minimum gating voltages to operate in favorable ranges.
Thus, in many regulated power supplies, charge pumps are used to amplify voltages. Charge pumps, supplied by a source voltage VDD, operate in a two-stage switched mode to provide an amplified voltage at an output. In a first phase of the charge pump""s operation, a capacitor is charged with a source voltage to the level of the source voltage. Then, on a second phase of the charge pump""s operation, the circuit is switched such that the source voltage and capacitor are connected in series to an output so as to create an amplified voltage at the output. The charge pump is capable of providing as much as twice the source voltage VDD at the output. Charge pumps may be employed as power supplies by driving output capacitors.
While the charge pump provides an increased voltage, the supplied voltage level varies depending on load variations and battery supply variations. When the load is relatively large and the battery supply is relatively low, the charge pump supplied voltage will be low and thus exhibiting the same problems as mentioned above. In contrast, when the load is relatively small the battery supply is relatively large, the charge pump supplied voltage may be too large, which can destroy oxide layers and otherwise reduce the lifecycle of the integrated circuit elements. Thus, regulating the output voltage of the charge pumps is important.
A known method of regulating the output voltage of a charge pump includes stacking diodes at the output of the charge pump to prevent the output voltage from exceeding a maximum voltage. When the output voltage of the charge pump reaches the turn-on voltage of the diode stack, current flows through the diode stack to ground. In low power applications, any current drain is undesirable. Therefore, while this technique prevents over-voltage conditions, it has the very undesirable side effect of increased power consumption and does not regulate the charge pump voltage for under-voltage conditions.
Another known method of regulating output voltage is disclosed in U.S. Pat. No. 4,223,238 which is incorporated by reference herein An oscillator provides true and complement oscillating output signals to a pair of conductors which are coupled to a charge pump for actively driving the charge pump. The output of the charge pump is coupled to an integrated circuit substrate for pumping charge into the integrated circuit substrate. A feedback circuit is coupled to the substrate as well for sensing the substrate bias voltage and for providing a control signal to a control input of the oscillator. When the magnitude of the substrate bias voltage exceeds a desired limit, the control signal output is switched so as to disable the oscillator. The true and complement outputs provided by the oscillator then assume a predetermined voltage such that the charge pump is no longer actively driven. The magnitude of the substrate bias voltage then decreases until the feedback circuit again enables the oscillator. This procedure is repeated periodically so as to maintain the substrate bias voltage at a desired point. A common implementation of the feedback circuit comprises a voltage comparator having a first input coupled to a given threshold voltage and a second input coupled across a voltage divider circuit connected to the integrated circuit substrate to sense the substrate bias voltage. The resistive or capacitive elements, however,. used to implement the voltage divider, increase the load seen at the output terminal of the regulated integrated circuit power supply, creating an undesirable effect of increasing power consumption.
Thus, a need exists for a regulated charge pump that does not increase power consumption.
A regulated integrated circuit power supply in accordance with the invention intermittently applies feedback to a charge pump on a sampled basis such that the charge pump is enabled to supply a voltage to an integrated circuit. Thereby, the regulated charge pump does not overload the integrated circuit coupled thereto. The regulated integrated circuit power supply includes a charge pump coupled to the integrated circuit to supply bias voltage. A feedback circuit senses the bias voltage and provides an output control signal based upon a comparison between the bias voltage and a voltage threshold. A switch connected to the feedback circuit selectively enables and disables sensing of the bias voltage. A signal generator provides at least one output signal. A controller receives both output signals from the signal generator and the feedback circuit to provide a first and a second control signal. The first control signal, responsive to the output signal of the signal generator, enables and disables the switch at predetermined intervals. The second control signal, however, responsive to both output signals, enables and disables the charge pump. Thus, since the substrate bias voltage is measured intermittently at predetermined intervals, the regulated integrated circuit power supply does not unnecessarily load the integrated circuit nor increase power consumption. A second embodiment may include a means for providing hysteresis.